Terephthalic acid (TPA) is one of the basic building blocks in the production of linear polyester resins used in the manufacture of polyester films, packaging materials, and bottles. TPA used in the manufacture of such polyesters resins must meet certain minimum purity requirements.
The purified condition of TPA refers primarily to the absence of significant concentrations of 4-carboxybenzaldehyde (4-CBA) and para-toluic acid (p-TAc) that are present in significant quantities in the commercially-available crude grades of TPA. Both 4-CBA and p-TAc are partial oxidation products formed in the manufacture of TPA by the catalytic oxidation of para-xylene. The purified form of TPA also refers to the absence of color bodies that impart a characteristic yellow hue to the crude material. The color bodies are aromatic compounds having the structures of benzils, fluorenones, and/or anthraquinones. 4-CBA and p-TAc are particularly detrimental to the polymerization process as they act as chain terminators during the condensation reaction between TPA and ethylene glycol in the production of polyethylene terephthalate (PET).
In a typical process for producing TPA, a slurry is withdrawn from the primary oxidation reactor. The slurry contains a liquid mother liquor and solid crude terephthalic acid (CTA) particles. The CTA particles are typically separated from the liquid mother liquor and then subjected to purification to thereby produce purified terephthalic acid (PTA). The separated mother liquor is typically treated to remove waste materials and then recycled to the primary oxidation reactor. Although most of the TPA produced in the primary oxidation reactor exists as solid CTA particles, a fraction of the TPA produced in the primary oxidation reactor is present in the liquid mother liquor. In conventional processes, the TPA in the liquid mother liquor represents a potential yield loss if it is not recovered and purified.
One conventional method of purifying CTA to produce PTA is by hydrogenation treatment, where 4-CBA is hydrogenated to p-TAc and color bodies to colorless solid compounds. To accomplish purification by hydrogenation, solid CTA particles are typically dissolved in a solvent (e.g., water), and the resulting solution is subjected to liquid-phase hydrogenation in the presence of a hydrogenation catalyst. Although effective to reduce yellowness, purification of CTA by hydrogenation can be expensive because it consumes energy, hydrogen, water, and catalyst. Hence, from the standpoint of operational cost, it would be desirable to minimize the amount of hydrogenation required to produce TPA solids of suitable purity.